The present invention relates to an image compression apparatus and, more particularly, to an image compression apparatus using a lossy compression method.
When a multi-contrast image constituted by a plurality of gray scale levels is to be compressed, a lossless compression method or a lossy compression method is normally used. As the lossless compression method, compression methods such as a run length coding method, a Huffman coding method, and the like, which can perfectly reproduce an original image are known. As the lossy compression method, a KLT method or a cosine transform method is known. The lossy compression method cam realize a higher compression ratio (a compression ratio of the lossless compression method: 1/2 to 1/3, a compression ratio of the lossy compression method: max. 1/10) than that of the lossless compression method. However, an original image cannot be perfectly reproduced, and the quality of the reproduced image is degraded in proportion to the compression ratio.
Of the lossy compression methods, a discrete cosine transform method receives a lot of attention since an image quality is not so degraded and a high-speed operation can be achieved. However, not only the discrete cosine transform method, but the discrete orthogonal transform method has the following disadvantages. For example, a so-called "block effect" phenomenon occurs, i.e., a block structure appears in a reproduced image due to an error caused when an image is divided into a plurality of blocks and when a low-frequency component is quantized. In addition, a "dim edge" phenomenon occurs, i.e., a clear edge cannot be obtained since a high-frequency component is cut off.